U.S. patent application number 16/796302 was filed with the patent office on 2021-08-26 for rf sensor heat shield.
The applicant listed for this patent is Raytheon Company. Invention is credited to Robert T. Manley, Waid A. Paine.
Application Number | 20210265717 16/796302 |
Document ID | / |
Family ID | 1000004685663 |
Filed Date | 2021-08-26 |
United States Patent
Application |
20210265717 |
Kind Code |
A1 |
Manley; Robert T. ; et
al. |
August 26, 2021 |
RF SENSOR HEAT SHIELD
Abstract
A radio-frequency (RF) heat shield for electronics includes a
first and second outer skin formed of an insulating material, an
insulating core layer arranged between the first and second outer
skin, wherein the insulating core layer has a lower dielectric
constant as compared with a higher dielectric constant of the first
and second outer skin, and a frequency selective surface (FSS)
layer including a reflective metallization pattern that is RF
transparent and formed on an exterior surface of each of the first
and second outer skin.
Inventors: |
Manley; Robert T.; (Allen,
TX) ; Paine; Waid A.; (Dallas, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Raytheon Company |
Waltham |
MA |
US |
|
|
Family ID: |
1000004685663 |
Appl. No.: |
16/796302 |
Filed: |
February 20, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01Q 1/002 20130101;
H01Q 1/42 20130101 |
International
Class: |
H01Q 1/00 20060101
H01Q001/00; H01Q 1/42 20060101 H01Q001/42 |
Claims
1. A radio-frequency (RF) heat shield for electronics, the RF heat
shield comprising: a first and second outer skin formed of an
insulating material; an insulating core layer arranged between the
first and second outer skin, wherein the insulating core layer has
a lower dielectric constant as compared with a higher dielectric
constant of the first and second outer skin; and a frequency
selective surface (FSS) layer including a reflective metallization
pattern that is RF transparent at a predetermined frequency range
and formed on an exterior surface of each of the first and second
outer skin.
2. The RF heat shield according to claim 1, wherein the higher
dielectric constant is between 3.0 and 4.5, the lower dielectric
constant is 1.4 or less, and a loss tangent of both the first and
second outer skin and the insulating core layer is 0.005 or
less.
3. The RF heat shield according to claim 1, wherein the insulating
material is quartz.
4. The RF heat shield according to claim 1, wherein the FSS layer
is formed of a pure metal material that is gold, silver, or
copper.
5. The RF heat shield according to claim 1, wherein the FSS layer
has a thickness that is between 0.02 and 0.2 microns.
6. The RF heat shield according to claim 1, wherein the reflective
metallization pattern has a plurality of spaced open regions and is
formed to cover between 75% and 95% of the exterior surface of each
of the first and second outer skin.
7. The RF heat shield according to claim 1, wherein the insulating
core layer is formed of a fibrous ceramic material.
8. The RF heat shield according to claim 1, wherein the first and
second outer skin and the insulating core layer are laminated to
each other.
9. The RF heat shield according to claim 1, wherein the insulating
core layer is formed by a vacuum canister, wherein the vacuum
canister contains a vacuum or air.
10. The RF heat shield according to claim 1 further comprising side
wall rings or stand-off posts that extend along the insulating core
layer between the first and second outer skin, wherein the side
wall rings or stand-off posts are formed of another insulating
material.
11. The RF heat shield according to claim 10, wherein the side wall
rings or the stand-off posts and the first and second outer skin
are formed of quartz.
12. The RF heat shield according to claim 10 further comprising
bond joints that are formed of a ceramic-based adhesive between the
side wall rings and the first and second outer skin.
13. The RF heat shield according to claim 10 further comprising
bond joints that are formed of a fused connection between the side
wall rings and the first and second outer skin, whereby a vacuum
enclosure is formed to define the core insulating layer.
14. The RF heat shield according to claim 10 further comprising an
inner skin formed of the insulating material of the first and
second outer skin, and wherein the insulating core layer is formed
of a first insulating core sublayer arranged between the first
outer skin and the inner skin and a second insulating core sublayer
arranged between the inner skin and the second outer skin.
15. A radome structure comprising: an exterior radome; an RF
antenna and corresponding sensor electronics arranged in the
external radome; and an RF heat shield arranged between the
exterior radome and the RF antenna and corresponding sensor
electronics, the RF heat shield comprising: a first and second
outer skin formed of an insulating material; an insulating core
layer arranged between the first and second outer skin, wherein the
first and second outer skin have a higher dielectric constant that
is between 3.0 and 4.5, the insulating core layer has a lower
dielectric constant that is 1.4 or less, and a loss tangent of both
the first and second outer skin and the insulating core layer is
0.005 or less; and a frequency selective surface (FSS) layer
including a reflective metallization pattern that is RF transparent
and formed on an exterior surface of each of the first and second
outer skin.
16. The radome structure according to claim 15, wherein the FSS
layer is formed of a pure metal that is gold, silver, or copper,
and wherein the reflective metallization pattern has a plurality of
spaced open regions and is formed to cover between 75% and 90% of
each of the exterior surface of the first and second outer
skin.
17. The radome structure according to claim 15, wherein the RF heat
shield includes side wall rings or stand-off posts that are formed
of another insulating material and extend along the insulating core
layer between the first and second outer skin.
18. The radome structure according to claim 15, wherein the
insulating core layer is formed as an enclosed vacuum chamber.
19. A method of forming an RF heat shield for electronics, the
method comprising: selecting an insulating material for outer skins
that have a dielectric constant between 3.0 and 4.5 and a loss
tangent that is 0.002 or less; selecting another insulating
material for an insulating core layer that has a lower dielectric
constant than the dielectric constant of the outer skins and a loss
tangent that is 0.005 or less; selecting thicknesses of the outer
skins and the insulating core layer to obtain a predetermined
frequency range; applying a reflective FSS metallization pattern
that is RF transparent at a predetermined frequency range on an
exterior surface of each of the outer skins using lithography and
metal deposition; and sandwiching the insulating core layer between
the outer skins.
20. The method according to claim 19 further comprising fusing the
outer skins to insulating side wall rings of a container defining
the insulating core layer thereby forming a vacuum in the
insulating core layer.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a heat shield radome arranged to
insulate sensor electronics from extreme external ambient
temperatures.
DESCRIPTION OF THE RELATED ART
[0002] Various applications require thermal insulation of
electronics that are subject to extreme temperature conditions,
including both hot and cold temperatures. Exemplary applications
include high speed aircrafts, missiles, re-entry vehicles, and
other space-based platforms having radio-frequency (RF) sensor
electronics that are subject to high external temperatures caused
by aerodynamic heating or other heat sources.
[0003] Prior attempts to provide a heat shield for sensor
electronics include using various materials as the heat shield
material. Exemplary prior materials include titanium dioxide
epoxy-filled paper and monolithic ceramic materials. However,
conventional heat shield arrangements are disadvantageous in that
the heat shields have a low thermal mass and will quickly equalize
in temperature with the surrounding temperature, such that the
shields effectively become a source of radiation heat transfer to
the sensor electronics. Moreover, prior attempts are not effective
at rejecting radiated heat and must absorb the majority of the
radiated heat.
[0004] One method used to compensate for low thermal mass includes
increasing the number of layers in the heat shield which
consequently increases the overall thickness of the heat shield.
However, increasing the thickness of the shield is disadvantageous
in that RF losses are increased and the overall sensor performance
is degraded.
SUMMARY OF THE INVENTION
[0005] An RF heat shield or radome described herein is used for
insulating electronics, such as an antenna and RF sensors, from
extreme external ambient temperatures. In exemplary applications,
the RF heat shield may be formed as a secondary radome arranged
within an exterior radome between the exterior radome and the
sensor electronics. In other exemplary applications, the RF heat
shield may be exposed without an exterior radome. The RF heat
shield is configured for both thermal insulation and low loss RF
transmission using two higher dielectric outer skins that sandwich
a lower dielectric, low loss tangent insulating core material that
provides low RF loss and a wide RF bandwidth. The described
configuration may be known as an A-sandwich construction. A
frequency selective surface (FSS) metallization pattern is formed
on the exterior surfaces of both outer skins to form an RF
transparent surface. While the forward low emissivity surface is
advantageous in rejecting radiation heat transfer from the external
environment into the RF heat shield, the aft FSS layer
advantageously impedes the ability of the RF heat shield to
re-radiate any radiation heat transfer from the RF heat shield to
the antenna and the sensor electronics. Thus, radiation heat
transfer to the sensor electronics is minimized.
[0006] The present application provides an RF heat shield having
improved insulation for a sensor and sensor electronics. The RF
heat shield is configured to insulate the sensor while minimizing
RF losses by rejecting 75% or more of the radiated heat which
minimizes the amount of insulating material required to protect the
sensor which minimizes the RF losses. The insulating core of the RF
heat shield may only absorb the remaining 25% that gets through the
FSS layer as compared with prior attempts in which 80% to 90% of
the radiated heat is absorbed. Advantageously, the insulating
effect is greatly improved without the need of adding excessive
amounts of insulating material which degrades RF performance.
[0007] The FSS metallization pattern is formed of a highly
reflective metal, such as polished gold, and has a pattern of
spaced openings that enable RF transmission. Forming the FSS
metallization pattern on the exterior surface of the radome skin is
advantageous as compared with conventional FSS surfaces that are
imbedded inside the radome skins which negates the emissivity
effect and enables heat transfer by conduction. The FSS may be
formed to have any suitable pattern and thickness such that the FSS
is able to support wide frequency bands of operation with low RF
loss.
[0008] The RF heat shield may be formed as an A-sandwich
construction formed of three layers of high-temperature resistant
materials. The layers are configured to provide low loss RF
transmission and high thermal insulation in extreme temperature
environments while also maintaining low RF losses. The outer skins
may be formed of quartz and the core material may be formed of a
low-density and fibrous ceramic insulator that provides low thermal
conductivity. Using an A-Sandwich configuration has the additional
advantage of allowing a very low dielectric and very low loss
tangent insulating material that can be made thicker which further
enhances the insulating properties of the heat shield with minimal
effect to RF losses.
[0009] The A-sandwich construction enables increasing the thickness
of the radome structure with additional low dielectric, low loss
tangent insulator material, increasing the thermal insulating
properties, and thermal mass without increasing the RF losses of
the radome. The thicknesses of the outer skins and the center core
can be tuned for the desired frequency of operation and desired
bandwidth, consistent with standard A-Sandwich radome
configurations. Additionally, the exterior FSS metallization
pattern rejects the majority of the radiation heat transfer
minimizing the insulation requirements of the internal core.
Advantageously, the thickness of the RF heat shield may be
increased while maintaining the same low RF losses as compared with
a conventional monolithic heat shield in which RF losses are
increased as the thickness of the shield is increased.
[0010] In another exemplary embodiment, the RF heat shield may have
a C-sandwich construction formed of five layers that are similar to
the layers of the A-sandwich construction. The C-sandwich
construction further includes an additional inner skin and
insulating core sublayers arranged between the inner skin and the
outer skins.
[0011] In an exemplary embodiment of the RF heat shield, the core
insulating layer may be formed as a vacuum chamber or air-filled
chamber between the radome skins. Forming the core insulating layer
as a vacuum chamber is further advantageous in minimizing the heat
transfer between the radome skins.
[0012] According to an aspect of the invention, an RF heat shield
for an antenna and sensor electronics includes outer skins, an
insulating core layer arranged between the outer skins, and an FSS
layer formed on an exterior surface of the outer skins.
[0013] According to an aspect of the invention, an RF heat shield
for an antenna and sensor electronics includes a forward low
emissivity surface that rejects radiation heat transfer from an
external environment into the RF heat shield, and an aft FSS layer
that impedes the ability of the RF heat shield to re-radiate any
radiation heat transfer from the RF heat shield to the
electronics.
[0014] According to an aspect of the invention, an RF heat shield
for electronics includes a first and second outer skin formed of an
insulating material, an insulating core layer arranged between the
first and second outer skin, with the insulating core layer having
a lower dielectric constant as compared with a higher dielectric
constant of the first and second outer skin, and an FSS layer
including a reflective metallization pattern that is RF transparent
at a predetermined frequency range and formed on an exterior
surface of each of the first and second outer skin.
[0015] According to an embodiment of any paragraph(s) of this
summary, the higher dielectric constant may be between 3.0 and 4.5,
the lower dielectric constant may be 1.4 or less, and a loss
tangent of both the first and second outer skin and the insulating
core layer may be 0.005 or less.
[0016] According to an embodiment of any paragraph(s) of this
summary, the insulating material may be quartz.
[0017] According to an embodiment of any paragraph(s) of this
summary, the FSS layer may be formed of a pure metal material that
is gold, silver, or copper.
[0018] According to an embodiment of any paragraph(s) of this
summary, the FSS layer may have a thickness that is between 0.02
and 0.2 microns.
[0019] According to an embodiment of any paragraph(s) of this
summary, the reflective metallization pattern may have a plurality
of spaced open regions and is formed to cover between 75% and 95%
of the exterior surface of each of the first and second outer
skin.
[0020] According to an embodiment of any paragraph(s) of this
summary, the insulating core layer may be formed of a fibrous
ceramic material.
[0021] According to an embodiment of any paragraph(s) of this
summary, the first and second outer skin and the insulating core
layer may be laminated to each other.
[0022] According to an embodiment of any paragraph(s) of this
summary, the insulating core layer may be formed by a vacuum
canister, with the vacuum canister containing a vacuum or air.
[0023] According to an embodiment of any paragraph(s) of this
summary, the RF heat shield may include side wall rings or
stand-off posts that extend along the insulating core layer between
the first and second outer skin, with the side wall rings or
stand-off posts being formed of another insulating material.
[0024] According to an embodiment of any paragraph(s) of this
summary, the side wall rings or the stand-off posts and the first
and second outer skin may be formed of quartz.
[0025] According to an embodiment of any paragraph(s) of this
summary, the RF heat shield may include bond joints that are formed
of a ceramic-based adhesive between the side wall rings and the
first and second outer skin.
[0026] According to an embodiment of any paragraph(s) of this
summary, the RF heat shield may include bond joints that are formed
of a fused connection between the side wall rings and the first and
second outer skin, with a vacuum enclosure being formed to define
the core insulating layer.
[0027] According to an embodiment of any paragraph(s) of this
summary, the RF heat shield may include an inner skin formed of the
insulating material of the first and second outer skin, with the
insulating core layer being formed of a first insulating core
sublayer arranged between the first outer skin and the inner skin
and a second insulating core sublayer arranged between the inner
skin and the second outer skin.
[0028] According to another aspect of the invention, a radome
structure includes an exterior radome, an RF antenna and
corresponding sensor electronics arranged in the external radome,
and an RF heat shield arranged between the exterior radome and the
RF antenna and corresponding sensor electronics. The RF heat shield
includes a first and second outer skin formed of an insulating
material, an insulating core layer arranged between the first and
second outer skin, with the first and second outer skin having a
higher dielectric constant that is between 3.0 and 4.5, the
insulating core layer having a lower dielectric constant that is
1.4 or less, and a loss tangent of both the first and second outer
skin and the insulating core layer being 0.005 or less, and an FSS
layer including a reflective metallization pattern that is RF
transparent at a predetermined frequency range and formed on an
exterior surface of each of the first and second outer skin.
[0029] According to an embodiment of any paragraph(s) of this
summary, the FSS layer may be formed of a pure metal that is gold,
silver, or copper, with the reflective metallization pattern having
a plurality of spaced open regions and being formed to cover
between 75% and 90% of each of the exterior surface of the first
and second outer skin.
[0030] According to an embodiment of any paragraph(s) of this
summary, the RF heat shield may include side wall rings or
stand-off posts that are formed of another insulating material and
extend along the insulating core layer between the first and second
outer skin.
[0031] According to an embodiment of any paragraph(s) of this
summary, the insulating core layer may be formed as an enclosed
vacuum chamber.
[0032] According to still another aspect of the invention, a method
of forming an RF heat shield for electronics includes selecting an
insulating material for outer skins that have a dielectric constant
between 3.0 and 4.5 and a loss tangent that is 0.002 or less,
selecting another insulating material for an insulating core layer
that has a lower dielectric constant than the dielectric constant
of the outer skins and a loss tangent that is 0.005 or less,
selecting thicknesses of the outer skins and the insulating core
layer to obtain a predetermined frequency range, applying a
reflective FSS metallization pattern that is RF transparent on an
exterior surface of each of the outer skins using lithography and
metal deposition, and sandwiching the insulating core layer between
the outer skins.
[0033] According to an embodiment of any paragraph(s) of this
summary, the method may include fusing the outer skins to
insulating side wall rings of a container defining the insulating
core layer thereby forming a vacuum in the insulating core
layer.
[0034] To the accomplishment of the foregoing and related ends, the
invention comprises the features hereinafter fully described and
particularly pointed out in the claims. The following description
and the annexed drawings set forth in detail certain illustrative
embodiments of the invention. These embodiments are indicative,
however, of but a few of the various ways in which the principles
of the invention may be employed. Other objects, advantages and
novel features of the invention will become apparent from the
following detailed description of the invention when considered in
conjunction with the drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0035] The annexed drawings, which are not necessarily to scale,
show various aspects of the invention.
[0036] FIG. 1 shows an RF heat shield for electronics arranged in
an exterior radome according to an exemplary embodiment of the
present application.
[0037] FIG. 2 shows a side view of the RF heat shield of FIG.
1.
[0038] FIG. 3 shows a construction of a vacuum canister formed of
quartz or a similar material.
[0039] FIG. 4 shows a quartz ring and two quartz skins that are
fused together in a vacuum to form a hermetic vacuum canister.
[0040] FIG. 5 shows an FSS metallization pattern formed on an
exterior surface of the outer skins of the RF heat shield of FIG. 1
according to an exemplary embodiment of the present
application.
[0041] FIG. 6 shows the FSS metallization pattern of FIG. 5
according to another exemplary embodiment in which the open regions
are formed as crosses.
[0042] FIG. 7 shows the FSS metallization pattern of FIG. 5
according to another exemplary embodiment in which the open regions
are formed as crosses with feet.
[0043] FIG. 8 shows the FSS metallization pattern of FIG. 5
according to another exemplary embodiment in which the open regions
are formed as crosses having different sizes.
[0044] FIG. 9 shows the FSS metallization pattern of FIG. 5
according to another exemplary embodiment in which the open regions
have a ring shape.
[0045] FIG. 10 shows the FSS metallization pattern of FIG. 5
according to another exemplary embodiment in which the open regions
have a y-shape.
[0046] FIG. 11 shows the FSS metallization pattern of FIG. 5
according to another exemplary embodiment in which the open regions
have a y-shape and are spaced more tightly as compared with the
open regions of FIG. 10.
[0047] FIG. 12 shows the FSS metallization pattern of FIG. 5
according to another exemplary embodiment in which the open region
is formed as a ring around the periphery of the outer skin.
[0048] FIG. 13 shows the RF heat shield having a C-sandwich
construction according to another exemplary embodiment of the
present application.
[0049] FIG. 14 shows a flowchart for a method of forming the RF
heat shield shown in any of FIGS. 1-13.
DETAILED DESCRIPTION
[0050] The principles described herein have particular application
in RF communication applications including antenna and radar
systems that contain sensors and sensor electronics. Suitable
platforms for the RF heat shield described herein include
aircrafts, space vehicles, high speed vehicles, or any other space
application in which a platform is exposed to extreme temperature
environments. Extreme temperature environments may include those in
which the platform is exposed to radiation from the sun or cold
temperature environments. The platform may be manned or unmanned
and defense applications or non-military applications may be
suitable. For example, missiles and satellites are both suitable
applications.
[0051] Referring first to FIGS. 1 and 2, an RF heat shield 20 may
be arranged in an exterior radome 22 as a secondary radome. An RF
antenna and corresponding sensor electronics 24 are arranged in the
exterior radome 22 and the RF heat shield 20 is arranged between
the sensor antenna and electronics 24 and the exterior radome 22.
The RF heat shield 20 is configured to thermally insulate the
sensor antenna and electronics 24 from radiation heat transfer from
the exterior radome 22 while also enabling forward RF transmission.
In exemplary applications, the RF heat shield 20 may be mounted
relative to the sensor antenna and electronics 24 without an
exterior radome, such as in a satellite application or in any other
application in which the sensor antenna and electronics are not
susceptible to damage by being exposed.
[0052] A radial mounting structure or canister 26 may be used for
mounting the RF heat shield 20 to the exterior radome 22 and is
also configured to provide thermal isolation from radial heat
transfer. A metal canister or frame may be provided as the radial
mounting structure 26. Any suitable mounting structure that
protects the sides of the sensor antenna and electronics 24 may be
provided and any suitable materials that provide electrical and
thermal insulation may be used to form the radial mounting
structure 26. Ceramic materials may be suitable. The radial
mounting structure 26 may be configured as a vacuum canister to
provide additional thermal insulation from radial heat transfer.
The vacuum canister may be configured with low emissivity surfaces
to further reject radiation heat transfer towards the sensor.
[0053] The RF heat shield 20 may have a planar arrangement that is
formed of a plurality of planar layers that are stacked relative to
each other and configured for both thermal and electrical
insulation. The layers in the planar arrangement may have any
suitable shape, such as rectangular or cylindrical. The RF heat
shield 20 may be shaped for integration with any conformal
structure that surrounds the antenna and electronics 24, such as a
vacuum canister. Other shapes may be suitable and the shapes may be
dependent on the application. In other exemplary embodiments, the
RF heat shield 20 may have a plurality of stacked layers formed in
a hemispherical shape. In any embodiment, the RF heat shield 20
includes outer layers or skins 28, 30 that extend parallel to each
other and an insulating core layer 32 that is interposed or
sandwiched between the outer skins 28, 30 to minimize heat transfer
between the outer skins 28, 30.
[0054] The outer skins 28, 30 may have a length L.sub.1 that is
greater than a corresponding length L.sub.2 of the sensor antenna
and electronics 24, such that the outer skins 28, 30 extend over an
entire surface of the sensor antenna and electronics 24. One outer
skin 30 of the outer skins 28, 30 is arranged proximate the sensor
antenna and electronics 24 and the other outer skin 28 is arranged
on an opposing side of the insulating core layer 32. The length
L.sub.1 of the outer skins 28, 30 defines the total length of the
RF heat shield 20 and the length L.sub.1 is elongated as compared
with a width W of the RF heat shield 20. The skins 28, 30 may have
different lengths. For example, forming the skin 30 to have a wider
diameter may be advantageous for attaching the RF heat shield 20 to
the mounting structure.
[0055] The outer skins 28, 30 are formed of any suitable material
that has a high temperature resistance, a low dielectric constant
that is higher than that of the insulating core layer 32, a low
loss tangent, and is able to withstand a metallization process. The
dielectric constant is defined as the ratio of the material
permittivity to the permittivity of vacuum, or the relative
permittivity. The loss tangent is defined as the ratio (or angle in
a complex plane) of the lossy reaction to the electric field to the
lossless reaction. Loss tangent is a measure of the dissipative
loss of RF fields as they propagate through dielectric materials.
For a dielectric material, an applied electric field causes the
polarization of atoms/molecules of the materials to create electric
dipole moments that create dissipation losses, i.e. heat, due to
damping of the vibrating dipole moments of the polarized
atoms/molecules that result in RF losses as electromagnetic fields
propagate through dielectric materials.
[0056] In an exemplary embodiment, any material having a
temperature resistance of up to 1200 degrees Celsius, a dielectric
constant that is between 3.0 and 4.5 (relative to vacuum), and a
low loss tangent that is less than 0.002 may be suitable. In an
exemplary embodiment, the material may have a dielectric constant
that is approximately 3.8 and a low loss tangent that is
approximately 0.001. The material may have an emissivity that is
between 0.4 and 0.95. Suitable materials for the outer skins 28, 30
include quartz or fused silica. Other materials may be suitable,
such as glass or other manufactured ceramic materials.
[0057] In an exemplary embodiment of the insulating core layer 32,
the insulating core layer 32 may be formed of a material having a
high temperature resistance, a low dielectric constant and a low
loss tangent, such as a temperature resistance of up to 1200
degrees Celsius, a dielectric constant that is 1.4 or less and a
low loss tangent that is 0.005 or less. The material dielectric
constant of the insulating core layer 32 is less than the
dielectric constant of the outer skins 28, 30. Using a low loss
tangent core material is advantageous in providing low RF losses
for the antenna and sensor electronics 24.
[0058] The material of the insulating core layer 32 may be a low
density and fibrous ceramic material having a low thermal
conductivity. For example, a silica ceramic material may be
suitable. The material may have a density that is between 0.2 and
0.9 grams per cubic centimeter and a thermal conductivity that is
between 0.02 and 0.20 watts per meter-Kelvin. The insulating core
layer 32 may have any suitable structure, such as a honeycomb type
structure or any other porous structure. A ceramic foam material
having a porosity that is between 50% and 95% may be suitable. The
material may further include silica aerogel or any other suitable
material to further lower the thermal conductivity of the
insulating core layer 32. Many other materials may be suitable for
the insulating core layer 32.
[0059] The insulating core layer 32 formed of the low dielectric,
low loss tangent material is advantageous in providing low RF
losses and wide RF bandwidths. Furthermore, the insulating core
layer 32 may be optimized for a desired frequency range by
adjusting the thickness of the outer skins 28, 30 and the
insulating core layer 32.
[0060] The RF heat shield 20 may be configured as an A-sandwich
type construction having the three layers formed by the skins and
the core. Forming the RF heat shield 20 as an A-sandwich
construction is advantageous in that the thickness of the layers
may be adjusted to increase the thermal mass of the RF heat shield
20 while maintaining low RF losses as compared with conventional
monolithic heat shields in which RF losses are increased as the
thickness is increased. A width of the insulating core layer 32 may
be greater than a width of each of the outer skins 28, 30. The
width of the insulating core layer 32 may be greater than twice the
width of the outer skin 28, 30. In an exemplary embodiment, the
thicknesses of the outer skins 28, 30 may be approximately two
millimeters or less and the thickness of the insulating core layer
32 may be four millimeters or more. The thicknesses of the skins
28, 30 and the insulating core layer 32 are frequency dependent and
may have any suitable thickness.
[0061] In another exemplary embodiment, the RF heat shield 20 may
be configured as a C-sandwich type construction that includes five
layers which are similar to those of the A-sandwich construction.
The outer skins 28, 30 are formed to be as thin as possible to
maximize the frequency bandwidth. The thickness of the outer skins
28, 30 may be formed based on structural requirements that
accommodate shock, vibration, and pressure loading. The frequency
band of operation typically drives the core thickness. Typically
the center core thickness is 10 to 20 times the thickness of the
skin thickness.
[0062] The RF heat shield 20 may further include an air gap 34
defined between each of the outer skins 28, 30 and the insulating
core layer 32. The air gap 34 may extend along the entire length
L.sub.1 of the outer skins 28, 30 and the air gap 34 may have a
thickness that is less than the thickness of the outer skins 28,
30. Providing the air gap 34 is advantageous in providing a further
electrical and thermal insulation effect. Many other thicknesses
are possible and the thicknesses are dependent on the
application.
[0063] The outer skins 28, 30 may be connected to side wall rings
36, 38 that extend along the thickness of the insulating core layer
32 between the outer skins 28, 30. The side wall rings 36, 38 may
extend parallel with each other and the outer perimeter of the RF
heat shield 20 may be defined by the outer skins 28, 30 and the
side wall rings 38, 38. The side wall rings 36, 38 may be formed of
any suitable insulating material and may form the canister, such as
the canister 26 shown in FIG. 1, that defines the insulating core
layer 32. Quartz or fused silica may be suitable materials for the
side wall rings 36, 38. The outer skins 28, 30 may be formed as
planar disks or any other suitable shape.
[0064] A bond joint 40 may be connected between each outer skin 28,
30 and a corresponding side wall ring 36, 38. Any suitable method
may be used to form the bond joint 40. For example, the bond joint
40 may be formed using a high temperature-resistant ceramic-based
adhesive material and adhering the outer skins 28, 30 to the
canister. A high temperature epoxy or cement may be used. The
epoxying process may be suitable for bonding the outer skins 28, 30
formed of quartz to the insulating material of the canister.
[0065] In another exemplary embodiment, the side wall rings 36, 38
may be machined and the outer skins 28, 30 may be laminated to the
outside surface of the side wall rings 36, 38. Many other bonding
materials and manufacturing methods may be suitable. In still other
exemplary embodiments, the RF heat shield 20 may include the outer
skins 28, 30 and the insulating core layer 32 without the side wall
rings and with the outer skins 28, 30 and the core insulating layer
32 being laminated.
[0066] In still other exemplary embodiment, the side wall between
the outer skins 28, 30 may be a continuous ring, or include
multiple stand-off posts to maintain a predetermined separation of
the outer skins 28, 30. The side wall may be formed of the same
material as the outer skins 28, 30, or any other suitable materials
that meet the structural requirements of the application. For
example, a higher density structural insulator material may be
suitable. The attachment may be formed by fusing the similar skin
and side wall (or stand-off post) materials to form the joint.
[0067] In another exemplary embodiment of the RF heat shield 20,
the insulating core layer 32 may be formed as a vacuum chamber or
an air-filled chamber between the outer skins 28, 30. Forming the
insulating core layer 32 as a vacuum chamber is advantageous in
further increasing the insulation effect of the insulating core
layer 32. In the exemplary embodiment in which the insulating core
layer 32 is formed as the vacuum chamber, the side wall rings 36,
38 define walls of the canister such that the canister is a vacuum
canister. The canister may be formed of quartz or any other
suitable material and both the canister and the outer skins 28, 30
may be formed of quartz. The vacuum in the insulating core layer 32
may be formed by fusing the outer skins 28, 30 to the side wall
rings 36, 38 at each bond joint 40.
[0068] FIGS. 3 and 4 show a construction of a vacuum canister 42
formed entirely of quartz or a similar material. The vacuum
canister 42 includes a quartz ring 42a and two quartz skins 42c,
42d that are fused together in a vacuum to form the hermetic vacuum
canister 42.
[0069] Referring now to FIG. 5, a frequency selective surface (FSS)
layer 44 including a reflective metallization pattern 46 that is RF
transparent is formed on an outermost or exterior surface 48 of
each of the outer skins 28, 30 opposite to the insulating core
layer arranged inside the outer skins 28, 30. While the forward low
emissivity surface is advantageous in rejecting radiation heat
transfer from the external environment into the RF heat shield 20,
forming the FSS layer 44 on the exterior surface 48 of the outer
skins 28, 30 is advantageous in that the aft FSS layer 44 provides
an RF transparent surface that impedes the ability of the RF heat
shield 20 to re-radiate any radiation heat transfer from the RF
heat shield 20 to the antenna and the sensor electronics. The low
emissivity effect is advantageous as compared with conventional FSS
surfaces that are imbedded inside the radome skins such that heat
transfer occurs by conduction.
[0070] The FSS layer 44 is formed of any suitable metal material,
such as gold, silver, or copper. Other pure, non-coated, and
reflective metals that are suitable for lithography may be used.
The FSS layer 44 is planar and may be formed by any suitable
manufacturing process, such as lithography and metal deposition
processes. A thickness of the FSS layer 44 is selected based on the
electrical requirements of the application and the thickness of the
FSS layer 44 is significantly less than the thickness of the outer
skins 28, 30. For example, the FSS layer 44 may have a thickness
that is between 0.02 and 0.2 microns. The FSS layer 44 is formed to
have a minimal thickness without the material, such as gold, being
translucent. The thickness of the FSS layer 44 is selected to
maximize an amount of the metal material surface area thereby
minimizing the emissivity of the FSS layer 44 to prevent heat
transfer radiation. For example, the FSS layer 44 may be formed to
provide an emissivity that is between 0.1 and 0.25.
[0071] In an exemplary embodiment, an adhesion metal deposition may
be applied between the gold layer and dielectric skin. The adhesion
metal deposition increases the adhesion strength between the gold
and dielectric skin to ensure that the gold metallization does not
delaminate under high temperature exposure due to the differing
coefficients of thermal expansion (CTE) of the gold and dielectric
skin material. Suitable materials include titanium tungsten,
titanium platinum gold, tantalum nitride or any other suitable
materials. A thickness of the adhesion layer is between 30 and 300
nanometers.
[0072] Referring in addition to FIGS. 6-12, any suitable reflective
metallization pattern 46 for the FSS layer 44 may be used and the
pattern may be dependent on the application. The metallization
pattern 46 may be formed to cover between 75% and 95% of the
surface area for the outer skin 28, 30, such that the pattern 46 is
formed to minimize the amount of open regions in which the
dielectric material of the outer skin 28, 30 is exposed. The amount
of coverage of the metallization pattern 46 is also selected to
enable a predetermined amount of RF passage such that the amount of
coverage is dependent on the electrical requirements of the
application. The sizes, dimensions, and spacing of the pattern 46
may be selected based on the frequency and scan angle of the
antenna.
[0073] The metallization pattern 46 may be an ordered pattern that
includes a periodic series of open regions or features in the metal
material. The open regions may be formed as slots, x's, crosses,
circles, squares, combinations thereof, or any other suitable
shapes. The open regions enable passage of RF through the FSS layer
44. FIG. 5 shows x's that are arranged in ordered columns and rows
along the surface area of the outer skin 28, 30. FIG. 6 shows
crosses 50 and FIG. 7 shows the crosses 52 having projecting feet
54 at the tops of the crosses 52. The open regions may have an
unordered pattern with different sizes and non-uniform spacings.
For example, FIG. 8 shows crosses 56 having different sizes such
that the spacing between the crosses 56 is also non-uniform.
[0074] FIGS. 9, 10, and 11 shows the open regions being formed as
other shapes. FIG. 9 shows the open regions being formed as rings
58. As shown in FIG. 9, the rings 58 may be non-continuous and have
a gap in the ring. FIG. 10 shows the open regions having a y-shape
60 and a predetermined spacing between the open regions. FIG. 11
shows the open regions having a y-shape and being spaced more
tightly as compared with the open regions of FIG. 10.
[0075] FIG. 12 shows still another embodiment of the FSS layer 44.
The exterior surface 48 of the outer skin 28, 30 includes the metal
layer 64 deposited in the center of the exterior surface 48 and the
open region is formed as a ring 66 that surrounds the metal layer
64. The arrangements of the FSS layer 44 shown in FIGS. 5-12 are
merely exemplary and many other shapes and patterns are
suitable.
[0076] Referring now to FIG. 13, in an exemplary embodiment, the RF
heat shield 20' may have a C-sandwich construction having five
layers, as compared with the A-sandwich construction having three
layers shown in FIGS. 1 and 2. The C-sandwich construction may also
be a planar arrangement having planar layers that are similar to
the layers of the RF heat shield 20 having the A-sandwich
construction. The layers in the C-sandwich arrangement include the
outer skins 28, 30, which are similar to the outer skins 28, 30 of
the RF heat shield 20 having the A-sandwich construction, and an
additional inner skin 68. The inner skin 68 is arranged in the
middle of the C-sandwich construction and is formed as a single
layer. The inner skin 68 may be formed of the same material as the
outer skins 28, 30 and the material may be any material previously
described as being suitable for the outer skins 28, 30. For
example, the outer skins 28, 30 and the inner skin 68 may be formed
of quartz or any material having similar properties to quartz.
[0077] The insulating core layer may be formed of a first
insulating core sublayer 32a arranged between the outer skin 28 and
the inner skin 68, and a second insulating core sublayer 32b
arranged between the inner skin 68 and the outer skin 30. The
insulating core sublayers 32a, 32b may be formed of the same
material or each insulating core sublayer 32a, 32b may be formed as
a vacuum chamber. The materials previously described as being
suitable for the insulating core layer 32 of the RF heat shield 20
having the A-sandwich construction are also suitable for the
insulating core sublayers 32a, 32b of the RF heat shield 20 having
the C-sandwich construction. For example, the insulating core
layers 32a, 32b may be formed of a low-density, fibrous ceramic
material. Forming the RF heat shield 20' to have a C-sandwich
construction having more layers than an A-sandwich construction may
be advantageous in particular applications for increasing the
insulation effect of the heat shield.
[0078] In an exemplary embodiment, the inner skin 68 may also have
a metallization pattern formed on the opposing faces 70, 72 of the
inner skin 68 that face the insulating core sublayers 32a, 32b. The
metallization patterns may be similar to any of the metallization
patterns 46 shown in FIGS. 5-12 and previously described. Any
suitable material or method may be used to form the metallization
pattern. Forming the metallization patterns on the inner skin 68
may be particularly advantageous in an embodiment in which the
insulating core sublayers 32a, 32b are formed as vacuum
chambers.
[0079] Referring now to FIG. 14, a flowchart showing a method 74 of
forming an RF heat shield for electronics is shown. The RF heat
shield 20, 20' having any of the features previously shown and
described may be formed using the method 74 and the RF heat shield
20, 20' may be arranged in the exterior radome 22 (as shown in FIG.
1) or exposed to the environment. A first step 76 of the method 74
includes selecting an insulating material for the outer skins 28,
30 (as shown in FIGS. 1 and 2). The material may have a dielectric
constant that is between 3.0 and 4.5 and a loss tangent that is
0.002 or less. For example, the material may be quartz.
[0080] A next step 78 of the method 74 includes selecting another
insulating material for the insulating core layer 32 (as shown in
FIGS. 1 and 2). The insulating material for the insulating core
layer 32 has a lower dielectric constant than the dielectric
constant of the outer skins and a loss tangent that is 0.005. The
lowest loss tangent possible may be used. For example, the material
of the insulating core layer 32 may be a low-density and fibrous
ceramic material that provides low thermal conductivity. The
insulating core layer 32 may be formed of a porous ceramic foam
material. The materials of both the outer skin 28, 30 and the
insulating core layer 32 are selected to temperature resistant. For
example, the layers 28, 30, 32 may have a high-temperature
resistance of up to 1200 degrees Celsius. Other operating
temperatures that are less than 1200 degrees may be suitable. A
core material with lower thermal conductivity, but higher loss
tangent may be used to optimize thermal performance over electrical
performance, or a core material with slightly higher thermal
conductivity to achieve the lowest possible loss tangent may be
used to optimize electrical performance.
[0081] A step 80 of the method 74 includes selecting thicknesses of
the outer skins 28, 30 and the insulating core layer 32 to obtain a
predetermined frequency range that is dependent on the application.
A step 82 of the method 74 includes applying a reflective FSS
metallization pattern 46 that is RF transparent on an exterior
surface 48 of each of the outer skins 28, 30 (as shown in FIG. 5)
using lithography and metal deposition. Other methods may be
suitable. Any suitable material and pattern, such as the patterns
shown in FIGS. 5-12, may be formed. For example, the FSS
metallization pattern 46 may be formed of pure gold and provide
coverage of the outer skin 28, 30 that is between 75% and 95%. The
percentage may be selected to optimize emissivity and lower the
electrical performance or optimize electrical performance and lower
the emissivity. Accordingly, lower coverage percentages, such as
around 50% may also be suitable. Other pure, reflective metal
materials may also be suitable.
[0082] After performing the metallization patterning, the RF heat
shield 20, 20' is assembled. A step 84 of the method 74 includes
sandwiching the insulating core layer 32 between the outer skins
28, 30. Step 84 may include fusing the outer skins 28, 30 to
insulating side wall rings 36, 38 of a container, in a vacuum,
defining the insulating core layer 32 (as shown in FIGS. 3 and 4).
Step 84 may include epoxying the outer skins 28, 30 to the side
wall rings 36, 38 or laminating the outer skins 28, 30 to the
insulating core layer 32. Step 84 may include fusing the outer
skins 28, 30 to stand-off posts formed of the same material as the
outer skins 28, 30.
[0083] The RF heat shield radome described herein is advantageous
in that the FSS metallization pattern being formed on the exterior
surfaces of the quartz radome skins forms an RF transparent surface
that rejects radiation heat transfer into the heat shield, such
that radiation heat transfer to the antenna and the sensor
electronics is minimized. Forming the FSS metallization pattern on
the exterior surface is particularly advantageous as compared with
conventional FSS surfaces that are imbedded inside the radome skin
and disadvantageously enable heat transfer by conduction. The aft
FSS minimizes radiation heat transfer between the RF heat shield
and the sensor antenna and electronics.
[0084] Using an A-sandwich construction is also advantageous in
that the insulating materials are configured to be resistant to
temperatures in extreme temperature environments, including hot and
cold temperatures, while also maintaining low RF losses. The
A-sandwich construction may be made thicker and have a higher
thermal mass for a same RF loss as compared with conventional
monolithic heat shields in which the RF losses are increased when
the number of layers is increased. Forming the insulating core
layer as a vacuum chamber may be advantageous in providing a
further insulation effect.
[0085] Still another advantage is that the RF heat shield may be
planar in construction to minimize reflection effects between the
heat shield and the exterior radome. The structure of the heat
shield enables the RF heat shield to be less complexly integrated
to various conformal structures surrounding sensor electronics,
such as a vacuum canister.
[0086] Although the invention has been shown and described with
respect to a certain preferred embodiment or embodiments, it is
obvious that equivalent alterations and modifications will occur to
others skilled in the art upon the reading and understanding of
this specification and the annexed drawings. In particular regard
to the various functions performed by the above described elements
(components, assemblies, devices, compositions, etc.), the terms
(including a reference to a "means") used to describe such elements
are intended to correspond, unless otherwise indicated, to any
element which performs the specified function of the described
element (i.e., that is functionally equivalent), even though not
structurally equivalent to the disclosed structure which performs
the function in the herein illustrated exemplary embodiment or
embodiments of the invention. In addition, while a particular
feature of the invention may have been described above with respect
to only one or more of several illustrated embodiments, such
feature may be combined with one or more other features of the
other embodiments, as may be desired and advantageous for any given
or particular application.
* * * * *